© 2006 Pearson Education

Chapter 3 Part 2

More about Strings

and

Conditional Statements

Loops (for and while)

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2

Repetition Statements

Repetition statements allow us to execute a statement multiple times

Often they are referred to as loops

Like conditional statements, they are controlled by boolean expressions

The text covers two kinds of repetition statements:• the while loop• the for loop

The programmer should choose the right kind of loop for the situation

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The while Statement

The while statement has the following syntax:

while ( condition ) statement;

while is areserved word

If the condition is true, the statement is executed.Then the condition is evaluated again.

The statement is executed repeatedly untilthe condition becomes false.

4

Logic of a while Loop

statement

true

conditionevaluated

false

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The while Statement

Note that if the condition of a while statement is false initially, the statement is never executed

Therefore, the body of a while loop will execute zero or more times

See Counter.java (page 147)

See Average.java (page 148)• A sentinel value indicates the end of the input• The variable sum maintains a running sum

See WinPercentage.java (page 151)• A loop is used to validate the input, making the program

more robust

while (value != 0) // sentinel value of 0 to terminate loop { count++;

sum += value; System.out.println ("The sum so far is " + sum);

System.out.print ("Enter an integer (0 to quit): "); value = scan.nextInt(); }

System.out.println (); System.out.println ("Number of values entered: " + count);

average = (double)sum / count;

DecimalFormat fmt = new DecimalFormat ("0.###"); System.out.println ("The average is " + fmt.format(average)); } }

while (won < 0 || won > NUM_GAMES) { System.out.print ("Invalid input. Please reenter: "); won = scan.nextInt(); }

ratio = (double)won / NUM_GAMES;

NumberFormat fmt = NumberFormat.getPercentInstance();

System.out.println (); System.out.println ("Winning percentage: " +

fmt.format(ratio)); } }

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Infinite Loops

The body of a while loop eventually must make the condition false

If not, it is an infinite loop, which will execute until the user interrupts the program

This is a common logical error

You should always double check to ensure that your loops will terminate normally

See Forever.java (page 152)

public class Forever { //----------------------------------------------------------------- // Prints ever-decreasing integers in an INFINITE LOOP! //----------------------------------------------------------------- public static void main (String[] args) { int count = 1;

while (count <= 25) { System.out.println (count); count = count - 1; }

System.out.println ("Done"); // this statement is never reached

} }

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Nested Loops

Similar to nested if statements, loops can be nested as well

That is, the body of a loop can contain another loop

Each time through the outer loop, the inner loop goes through its full set of iterations

See PalindromeTester.java (page 155)

while (another.equalsIgnoreCase("y")) // allows y or Y { System.out.println ("Enter a potential palindrome:"); str = scan.nextLine();

left = 0; right = str.length() - 1;

while (str.charAt(left) == str.charAt(right) && left < right) { left++; right--; }

System.out.println();

if (left < right) System.out.println ("That string is NOT a palindrome."); else System.out.println ("That string IS a palindrome.");

System.out.println(); System.out.print ("Test another palindrome (y/n)? "); another = scan.nextLine(); } } }

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Conditional Operator

Conditional operators are a shortcut way to express an if-else situation.

Format:

result = (condition)? value1 : value2;

“if” part

“else” part

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Conditional Operator

Example: (orig if-else)

if (x > 0) count = count + 1;else count = count – 1;

Example: (cond. operator)

count =(x > 0)? (count + 1) : (count – 1);

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The Do-While Loop

The do-while loop works like the while loop, BUT in a different order. The do portion is done first, before the while condition is checked.

Format: do{ … statements for what to do

}while (condition);

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Example of Do-While loop

int count = 0; int sum = 0;

do

{

sum = sum + count;

count ++;

}

while (count < 10);

Notice the while condition is not

checked until the do loop is executed at

least once.

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The for Statement

The for statement has the following syntax:

for ( initialization ; condition ; increment ) statement;

Reservedword

The initializationis executed once

before the loop begins

The statement isexecuted until the

condition becomes false

The increment portion is executed at the end of each iterationThe condition-statement-increment cycle is executed repeatedly

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The for Statement

A for loop is functionally equivalent to the following while loop structure:

initialization;while ( condition ){ statement; increment;}

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Logic of a for loop

statement

true

conditionevaluated

false

increment

initialization

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The for Statement

Like a while loop, the condition of a for statement is tested prior to executing the loop body

Therefore, the body of a for loop will execute zero or more times

It is well suited for executing a loop a specific number of times that can be determined in advance

See Counter2.java (page 161)

See Multiples.java (page 163)

See Stars.java (page 165)

public class Counter2 { //----------------------------------------------------------------- // Prints integer values from 1 to a specific limit. //----------------------------------------------------------------- public static void main (String[] args) { final int LIMIT = 5;

for (int count=1; count <= LIMIT; count++) System.out.println (count);

System.out.println ("Done"); } }

final int PER_LINE = 5; int value, limit, mult, count = 0; Scanner scan = new Scanner (System.in); System.out.print ("Enter a positive value: "); value = scan.nextInt(); System.out.print ("Enter an upper limit: "); limit = scan.nextInt(); System.out.println (); System.out.println ("The multiples of " + value + " between " + value + " and " + limit + " (inclusive) are:");

for (mult = value; mult <= limit; mult += value) { System.out.print (mult + "\t");

// Print a specific number of values per line of output count++; if (count % PER_LINE == 0) System.out.println(); } }

public class Stars { //----------------------------------------------------------------- // Prints a triangle shape using asterisk (star) characters. //----------------------------------------------------------------- public static void main (String[] args) { final int MAX_ROWS = 10;

for (int row = 1; row <= MAX_ROWS; row++) { for (int star = 1; star <= row; star++) System.out.print ("*");

System.out.println(); } } }

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The for Statement

Each expression in the header of a for loop is optional

• If the initialization is left out, no initialization is performed

• If the condition is left out, it is always considered to be true, and therefore creates an infinite loop

• If the increment is left out, no increment operation is performed

Both semi-colons are always required in the for loop header

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Choosing a Loop Structure

When you can’t determine how many times you want to execute the loop body, use a while statement

If you can determine how many times you want to execute the loop body, use a for statement

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Program Development

We now have several additional statements and operators at our disposal

Following proper development steps is important

Suppose you were given some initial requirements:

• accept a series of test scores

• compute the average test score

• determine the highest and lowest test scores

• display the average, highest, and lowest test scores

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Program Development

Requirements Analysis – clarify and flesh out specific requirements• How much data will there be?

• How should data be accepted?

• Is there a specific output format required?

After conferring with the client, we determine:• the program must process an arbitrary number of test

scores

• the program should accept input interactively

• the average should be presented to two decimal places

The process of requirements analysis may take a long time

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Program Development

Design – determine a possible general solution

• Input strategy? (Sentinel value?)

• Calculations needed?

An initial algorithm might be expressed in pseudocode

Multiple versions of the solution might be needed to refine it

Alternatives to the solution should be carefully considered

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Program Development

Implementation – translate the design into source code

Make sure to follow coding and style guidelines

Implementation should be integrated with compiling and testing your solution

This process mirrors a more complex development model we'll eventually need to develop more complex software

The result is a final implementation

See ExamGrades.java (page 170)

max = min = grade;

// Read and process the rest of the grades while (grade >= 0) { count++; sum += grade;

if (grade > max) max = grade; else if (grade < min) min = grade;

System.out.print ("Enter the next grade (-1 to quit): "); grade = scan.nextInt (); }

// Produce the final results if (count == 0) System.out.println ("No valid grades were entered."); else { DecimalFormat fmt = new DecimalFormat ("0.##"); average = (double)sum / count; System.out.println(); System.out.println ("Total number of students: " +

count); System.out.println ("Average grade: " +

fmt.format(average)); System.out.println ("Highest grade: " + max); System.out.println ("Lowest grade: " + min); } }

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Program Development

Testing – attempt to find errors that may exist in your programmed solution

Compare your code to the design and resolve any discrepancies

Determine test cases that will stress the limits and boundaries of your solution

Carefully retest after finding and fixing an error

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More Drawing Techniques

Conditionals and loops can greatly enhance our ability to control graphics

See Bullseye.java (page 173)

See Boxes.java (page 175)

See BarHeights.java (page 177)

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Summary

Chapter 3 has focused on:

• program development stages• the flow of control through a method• decision-making statements• expressions for making complex decisions• repetition statements• drawing with conditionals and loops

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Assignment 3 COMPLETE ALL ASSIGNMENTS IN THE ORDER LISTED Lab 2 -- Grades Lab 3 Phase 1

• Counting and Looping – Love CS• Powers of 2 – Powers• A guessing Game – Guess

Chapter 3 Exercises continued• numbers 9- 13

Lab 3 Phase 2• Factorials – Factorial (Ex. Credit)• More Guessing – GuessDo

Chapter 3 Exercises continued• numbers 14 - 22

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Assignment 4 COMPLETE ALL ASSIGNEMENTS IN THE ORDER LISTED Lab 4 Phase 1

• Counting Characters – CharCount Chapter 3 Programming Projects (pp. 187 – 188)

• 3.1, 3.4, 3.5, 3.7, 3.8, 3.12 a, b, 3.13 Lab 4 Phase 2

• Finding Maximum & Minimum Values – MaxMin Chapter 3 Exercises Continued

• numbers 23 – 27 Chapter 3 Exercises from textbook pp. 189

• 3.15 – 3.17 (Ex. Credit) Lab 5

• A Rainbow Applet – Rainbow (Ex. Credit)